In recent years, stepping motors which are based on old, well-known ideas have obtained extensive use in connection with the technical development of both motor design itself and its current supply. The basic principles for the stepping motor and its drive, as well as at least certain further developments are well-known to one skilled in the art and are therefore not described here. One not skilled in the art is referred to the book "STEPPING MOTORS: a Guide to Modern Theory and Practice" by P. P. Acarnley, published by Peter Peregrinius Ltd. for the Institution of Electrical Engineers as No. 19 in IEE Control Engineering Series.
Microstepping is a method well known to one skilled in the art for providing a stepping motor with greater resolution than full stepping. Microstepping is so well-known in conjunction with stepping motors that it should not be necessary to describe it here. Further references for one not skilled in the art are "Ministepping Motor Drives" by E. K. Pritchard, Proceedings of the Fifth Annual Symposium on Incremental Motion Control Systems and Devices, University of Ill.; TECHNOLOGY OF MICROSTEPPING; OEM DESIGN, October 1983, pp 69-71; and "Microstepping: Small Steps Turn Into Big Improvements" by E. Slingland, POWER CONVERSION INTERNATIONAL; October 1983, pp 20-25.
There are many drive circuits in the prior art for supplying current to stepping motors from voltage sources and controlling the phase currents through the stator windings. The basic principles for such drive circuits and such control are well-known to one skilled, in the art and are therefore not described here. One not skilled in the art is referred to such as the first and the last of the above-mentioned references. In microstepping a stepping motor, a current in a winding is to be controlled towards a variable desired value. The desired value can assume a plurality of different values and at least occasionally rapidly increase or rapidly decrease by successively assuming ever greater values or ever smaller values.
Certain known methods and controlling apparatus have not managed to control sufficiently accurately the stator current towards its desired value in certain operational cases. In certain cases the current has not decreased sufficiently rapidly in time with the desired value, and in other cases the current has oscillated heavily, alternating above and below the desired value so that an excessive current ripple has been obtained.
Problems have therefore occurred in the operation of stepping motors, particularly with high demands on speed, acceleration and retardation, in spite of modern motor design and modern drive circuits. The problems have taken the form of vibrations, dissonance and deviations between desired and actual positions. An invention aiming at the solution of, or at least the reduction of such problems in controlling current to a stator winding, particularly in conjunction with the microstepping of stepping motors, is described in the European patent application No. EP 0242344, the content of which is incorpated herein as a reference.
Expressed somewhat summarily and simplified, it could be said that a method in accordance with EP 0242344 is applicable in drive circuits which are controllable into at least three different operational states, normally signifying in a first state the maintenance or increase of the current, a second state with comparatively slow current decrease and a third state with comparatively rapid current decrease. Of a method in accordance with EP 242344 it could then be said in short that, inter alia, the time sequence of the desired value is sensed and that the possibility of using the third state in response to the result of the sensing is limited to certain sections of the desired value time sequence. There is thus obtained a better possibility of sufficiently rapidly and accurately regulating the current towards a varying desired value so that within reasonable limits the current has time to decrease or increase in time with rapidly decreasing or increasing desired values without excessive current ripple. Audible dissonance with vibrations, such as mechanical resonance from the motor and connected parts in the system, can be reduced or completely eliminated in certain operational cases. Furthermore, a higher speed can be achieved with the motor. In certain cases the motor attenuation can also be improved.
A controlling apparatus according to EP 0232344 includes, inter alia, sensing means for sensing the time sequence of the desired value and control restricting means adapted in response to the sensed time sequence of the desired value to limit the possibilities of controlling the drive circuits such as to enter into the third operational state. The control restricting means are also preferably arranged to restrict, in response to the sensed time sequence of the desired value, the possibilities of controlling the drive circuit into the second operational state.
According to the embodiments described in EP 0242344, a method and controlling apparatus according to EP 0242344 signify that solely or substantially the first and second operational states are utilized during the portions of the time sequence of the desired value when the absolute value of the value successively increases and at least the first and the third operational states, preferably the second also, are used during certain portions of the time sequence of the desired value when the absolute value of the latter successively decreases.
The different operational states of the drive circuit, and these can be more than three, are achieved according to EP 0242344 by controlling different combinations of current switches in the drive circuit into conducting and/or blocking states. The drive circuit can be, at least partially, of a conventional type and the three or more combinations of conductive and blocking states of these current switches can be of a type already known per se.